Ed,
It looks to be one of the Thorpe single cam engines. This is a slightly different one than appeared in the 1995 Mar/Apr issue of the New ConRod magazine, in that this has the later valve enclosure patterned upon the JAP ohv engines, and as used on the final Thorpe twin-cam engine. The earlier mentioned engine had Thorpe's alloy heads with slightly down-facing exhaust ports and hairpin valve springs like this, but used the detachable rocker perches from the 1931-35 Douglas ohv models, all exposed. While there was a Thorpe outfit racing from 1936 onward, casting of special heads and barrels date immediately postwar.
This is not however the Thorpe outfit but a highly modified Dougie (unknown) using a secondhand Thorpe engine. If it was sprinting into the seventies, someone out there will know more details about its history, and who constructed it.
-Doug